The story so far: Now in the second week of his detention, Spethmann awaits the resumption of Lychev's interrogations. In the meantime, he makes a breakthrough in his analysis of Rozental, the tormented chess genius about to take part in the forthcoming tournament. At least, so he thinks.

St Petersburg, 1914

Three more days passed. They were not entirely wasted. From the beginning of my incarceration, I had been allowed to receive books from Minna. Among these was the Babylonian Talmud, sent at my request. (It amused me that when we played chess, the old jailer would eye the sacred text suspiciously, uncertain whether it was safe to be in the same room with so potent a token of alien magic.)

I searched the texts from top to bottom, occasionally mumbling to myself as I read, which the jailer, looking in at the observation slit, took as prayer. My father would have been horrified at the sight, even had I been able to reassure him my reading had nothing to do with the God he rejected but with the patient I was concerned to save.

My father would have been embarrassed to have met Rozental. He was too much like the man from Dvinsk my father wanted to forget he had ever been. Kopelzon weeps rivers of sentimental tears when he talks of the poverty of the towns of the Pale. But my father's heart was not moved by the destitution of his people. He was shamed by it, as a son would be shamed by his father falling over drunk in the street. The sight of the schnorrer humiliated him; the soup kitchen he felt a personal disgrace. He asked himself a simple question: why were his people so miserably poor and ignorant? Why was there so much vice, prostitution and robbery? Why, among his people, were there so many terrorists and revolutionaries? Kopelzon would have answered him plainly with the words pogrom, Cossack, Pale and the Black Hundreds. But all my father saw was ignominy and backwardness.

My father would never have engaged in debate, with Kopelzon or anyone else. He fell silent when faced with the strong and contrary opinion of others, not so much because his lack of education left him ill-equipped to defend his point of view, but because he took his own beliefs as self-evidently true and therefore in no need of public airing. The answer he had discovered resided in his own people, in their very being. As long as his people were themselves, it could be not be otherwise. His success in the world only reinforced his belief. He had the answer and the answer was to stop being Jewish.

'What is on your head?' I had asked Rozental during a session early in our analysis. He had been scratching his scalp, clawing at it with the nails of both hands.

He uttered a groan. 'It never leaves me alone.'

His hair was very short, practically shorn. I examined it carefully. There was nothing, not even nits.

'It's a fly,' he said desperately. 'Can you not see it? It follows me everywhere. It torments me day and night.'

'There's nothing here.'

'I can feel it crawling over my scalp.'

'Would you like a mirror so you can see for yourself?'

With some difficulty, I induced him to stand before the mirror over the fireplace while I held a hand mirror (borrowed from Minna) behind him, as a barber does. Eventually, I settled him sufficiently to be able to continue the session.

My approach with my patients was always the same: I asked for as full an account as possible of their life story. Rozental began by describing his early years in Choroszcz, the destitute settlement in which he had lived until going to yeshiva in Lodz. He was the youngest of 12 children whose father had died before he was born. I asked how his mother had managed.

'My brothers and sisters and I were parcelled out to relatives. I was sent to live with my grandparents.'

'Tell me about your grandparents.'

He hesitated, then said: 'They were good, kind people, very simple.'

'Were they religious?'

'Yes, of course.'

'Were you happy in their house?'

'They loved me very much.'

Besides the obvious incompleteness of the answer, I thought I detected a trace of guilt. Here, plainly, was an avenue to explore. 'Are your grandparents still alive?'

'They are both dead.'

'Were they still alive when you became famous as a chess player?' His reluctance to answer confirmed me in my view that there was something of significance in his relationship with his grandparents. 'Avrom,' I prodded my taciturn patient, 'did they live long enough to hear of your successes?'

'Yes,' he whispered, almost inaudibly.

'How did they react?' He began again to scratch his scalp. 'Were they pleased?'

'How could they?' he retorted, this time forcibly and without vacillation. 'When they sent me to the heder, they said, "Learn, Avrom, learn! Purses of silver will fall to you from heaven." But instead of learning, what did I do? I played chess. Haran, Padan, Hebron where Abraham buried Sarah? None of this mattered - I was consumed by chess. When the boys were imagining themselves following Moses out of Egypt or fighting with Joshua at Jericho, I had visions of myself a pawn up against Lasker in a rook endgame. How could my grandparents have approved?'

'Were there arguments?' He did not answer, though I put the question three times. 'Do you feel you disappointed them?' I ventured.

'I just want to play chess!' he burst out. 'I ask nothing of anyone - nothing! I do not interfere with anyone, I do not criticise, I do not condemn. Why cannot I be left alone to play chess? Why?'

'Who is not leaving you alone?'

'Everyone.'

'Your grandparents?'

'Everyone wants me to be this, to be that. To do this, to do that.'

'What did your grandparents want you to be?'

'It's not my grandparents, it's not them!' he insisted desperately, though unconvincingly.

'Then who? Who has these expectations which you find so onerous?'

No matter how hard I probed, he would not be specific. It was everyone

'I want to play chess,' he sobbed. 'I want to play in the tournament. I want to play Lasker and beat him. But they won't let me!'

'They are your grandparents, Avrom, are not they not?'

Rozental answered by violently swatting the air around his head. The invisible fly had returned.

The fly was obviously the key to understanding Rozental's illness. But it was not until I was in my cell that I discovered its true symbolic meaning. Between Lychev's interrogations, my reading of the ancient texts led me to this: Beelzebub, the devil, got his name from ba'al-zevuv, meaning 'master of the fly'.

Searching further: 'Man scarcely dies when he is taken possession of and questioned by the demons. The demon Nesosh comes in the form of a fly, places himself upon the head and beats him mercilessly. The soul, separated from the body, arrives then at the bridge Tchinevad, which separates our world from the invisible world; there, it is judged by two angels, one of whom is Mithra, of colossal proportions, with 10,000 eyes, and holding a club in his hand.' And, most significantly, I came upon a midrash which runs: 'The evil inclination is similar to a fly and sits at the two openings of the heart.'

For someone as thoroughly steeped in Jewish learning and tradition as Rozental, the fly was a manifestation of the yetzer hara, of evil inclination, the impulse to follow selfish desire. It was not difficult to work out the nature of this particular evil inclination. Rozental had told me that, like Benjamin, the beloved youngest son of Jacob, he had been his grandparents' favourite. When I pressed him on his feelings for his father's parents, however, his replies were highly suggestive, for while he never criticised them, never once did he make a positive declaration in their favour.

He would repeat, almost formulaically, that they were 'good people', 'kind people', 'simple people'. But they had also been exacting of the promising young student and had entertained ambitions of him both as the economic saviour of the distressed family and as a future religious leader of their community. It was clear that he found these expectations too burdensome to bear. ('Everyone wants me to be this, to be that. To do this, to do that.' These words, I subsequently discovered, had an even greater significance than I believed at the time.)

Instead of fulfilling his grandparents' dreams, Rozental became obsessed with chess and quickly aspired to become not a great religious teacher but a great professional chess player. Whenever I asked him about his grandparents' reaction, Rozental's narrative faltered. Since professional chess would have taken him from their world and their religion, I decided the grandparents were unlikely to have approved, and that Rozental's reluctance to acknowledge this was because of residual feelings of loyalty. In spite of coming up against what must have been very strong emotional pressure, Rozental had found the courage to pursue his dream.

But in every corner demons lie in wait for the Jewish soul. Guilt had caught up with Rozental, precipitating his terrible mental crisis. I determined that on my release - assuming I would sooner or later be set at liberty - I would have to bring my patient to confront his true feelings for his grandparents.

This was the analysis at which I arrived during my imprisonment. I was both proud and certain of my deductions. At some future time, I would write a paper for presentation to my colleagues.

I had no idea how completely wrong I was.

More time. And then, at last, Lychev returned. In the blue, pre-dawn light filtering through the high barred window, Lychev's appearance was cyanotic. It had not been the first time I had wondered about the state of his health: a bad heart, I concluded, and time was to tell that in this at least I was not mistaken.

'I want to see my daughter,' I told him, rousing myself from the cot. Mentally, I had been preparing for this confrontation and had decided that instead of doing my best to co-operate, I should counterattack, however meagre my resources. 'I will answer no further questions until I have seen Catherine. I demand to know what charge you have against me. I demand to speak to a judge or lawyer.'

'Your daughter was released last night,' the detective said apathetically. I saw now the air of dejection about him. 'Gather your things. You, too, are being set free.' I was confounded. How I wanted to believe him and yet at the same time I could not be but suspicious - what if it were a trick to lower my guard? But then the old jailer appeared at the door. 'If your honour would follow me,' he said.

'What is going on?' I said.

'Your friends in high places have not forgotten you,' Lychev replied.

Zinnurov, Anna's father, must have intervened. Still scarcely believing this sudden turn of events, I collected my books and my few belongings and followed Lychev and the jailer down the dimly lit corridor. We passed into a large room with a low, concave ceiling and pillars and arches of brick. From here, I was escorted up a flight of stone steps into a courtyard. The air was frigid, but it was the cold of early spring, not winter, and heartening for that.

We came to a broad, squat gatehouse.

'Your friends may have prevailed upon Minister Maklakov to have you released,' Lychev said as the bolts were pulled back, 'but don't think you have seen the last of me. I will find Yastrebov's associates and you will help me, whether you like it or not.'

Lychev seemed slighter, more insubstantial than ever. I ignored him as I stepped outside, as one ignores a bore at a party. The instant the gate closed behind me, I had the sensation it had all been a dream.

Catherine emerged from the back of a large motor carriage and ran towards me. 'You're safe,' she said, throwing her arms about my neck.

She led me briskly to the waiting car. I could not believe her alacrity. A week ago it was unthinkable she would go anywhere near a car belonging to the Mountain. She had made her first compromise. To my surprise, I felt disappointed.

As we approached, a man in a beautifully tailored dark suit got out and took off his hat. 'Good morning, Doctor,' he said.

I stopped in my tracks. There had been no compromise. It was not the Mountain, but his nemesis. It was Gregory Petrov.

Next week: Catherine and her lover

· With acknowledgement to Grandmaster Nigel Davies and his 1992 article in the Jerusalem Post, 'Master of the Fly'.